The isocyanate is a compound having at least one isocyanate group (—NCO) and is industrially widely used as a raw material for polyurethane or polyurea.
Conventionally, isocyanates are industrially produced by reaction between amine and phosgene (phosgene method). There are, however, various disadvantages such that phosgene is hard to deal with due to its highly toxic nature and also by-produces hydrochloric acid in large quantity, so that corrosion of systems needs to be considered. Accordingly, it is desirable to develop an alternative method for industrially producing isocyanates.
As a method for producing isocyanates without using phosgene, for example, a method (carbonate method) of forming amines into carbamates using dialkyl carbonate and then thermally decomposing the obtained carbamates; and a method (urea method) of forming amines into carbamates using urea or N-unsubstituted carbamic acid ester and then thermally decomposing the obtained carbamates are known.
According to these methods, it is possible to produce carbamates without using phosgene and then decompose them into isocyanate and alcohol.
More specifically, there has been proposed, for example, a method for producing isocyanates and alcohol, which includes allowing organic polyamines, urea, and alcohols to react in the presence of dialkyl carbonates and alkyl carbamates to give polyurethanes (carbamates), removing the alcohol, the dialkyl carbonates, and the alkyl carbamates from the reaction mixture containing the obtained polyurethanes (carbamates), and then recycling the resultants into the reaction step of organic polyamines, urea, and alcohols, and thermally decomposing the reaction mixture containing the obtained polyurethanes (carbamates) (see, for example, the following Patent Document 1).
According to this method, polyurethanes (carbamates) are obtained as main products by allowing organic polyamines, urea, and alcohols to react, and simultaneously, dialkyl carbonates and alkyl carbamates obtained as by-products are recycled into the reaction step of organic polyamines, urea, and alcohols, so that the dialkyl carbonates and the organic polyamines are allowed to react, to thereby separately produce polyurethanes (carbamates).
The polyurethanes (carbamates) thus produced are thermally decomposed to thereby produce isocyanates without using phosgene.
It is, however, known for the carbonate method or the urea method that the carbamates or isocyanates produced, or intermediates thereof cause unpreferable polymerization reaction such as multimerization, biuretization, or allophanatization.
In this case, when the isocyanates and the alcohols are separated and recovered from the decomposition solution of carbamates, for example, by-products such as urea derivatives (biuret derivatives) or carbamate derivatives (allophanate derivatives) are obtained as isocyanate residues.
Although the phosgene method allows isocyanates to be produced by reaction between amine and phosgene, the isocyanates cause unpreferable polymerization reaction such as multimerization with this method, resulting in by-production of multimers or the like as isocyanate residues.
Such isocyanate residues are usually disposed of. In recent years, however, it is required that wastes should be reduced from the viewpoints of global environment and related factors, so that various methods for effectively using recovered isocyanate residues are studied. As one of such methods, there has been proposed, for example, a method for continuously feeding residues, which are by-produced when an isocyanate compound is produced by phosgene method, in a molten state or a solution state to a reactor as well as continuously feeding high-pressure/high-temperature water to the reactor, and setting the temperature in the reactor to 190 to 300° C., so that the residues are decomposed into polyamines to be recovered (see, for example, the following Patent Document 2).
In addition, carbonates such as dialkyl carbonate, diaryl carbonate, and alkyl aryl carbonate are used as various industrial materials, and they are known as by-products obtained when carbamates are produced by industrially allowing amine, urea and/or N-unsubstituted carbamic acid ester, and alcohol to react (carbamate formation reaction) as described above.